Sheet metal is used to form the basic components of many commercial products. For example, sheet metal is used to form parts for automobiles, appliances, airplanes and other mass produced items. To transform sheet metal into an appropriately sized and shaped part, a sheet metal work piece must be pressed, bent, cut, pierced, trimmed, etc.
A transfer press is typically used to expedite the process of forming parts from sheet metal. Transfer presses often include several upper and lower die sets or combinations that are arranged in a line within the transfer press. The die sets or die combinations are referred to as press stations. The dies for each press station are chosen to perform specific functions to create a desired part from a work piece. The transfer press generally includes an automated system to transfer the work piece from one press station to the next to increase the rate of output by the transfer press.
Over the years, the size of parts formed from sheet metal has increased significantly. For example, individual parts for automobiles such as doors and body panels have increased in size. Large parts typically slow down a conventional transfer press thus decreasing its output capability. Generally, it takes longer to move a large part between adjacent press stations. Additionally, large parts make it more difficult to reorient each work piece between dies because larger parts are more difficult to handle.
Prior systems and methods for transferring a work piece in a multiple station transfer press have used independent vertical and horizontal movement of a cross bar assembly. This independent vertical and horizontal movement frequently limited the rate at which large work pieces could be processed. Other systems use simultaneous vertical and horizontal movement of a cross bar assembly to increase the output of the associated transfer press. This type of movement is shown by way of example in U.S. Pat. No. 5,148,697 issued to Shiraishi, et al. entitled "Method for Withdrawing Work Piece From Drawing Mold" and U.S. Pat. No. 4,981,031 issued to Schneider, et al. entitled "Transfer Device in a Transfer Press or Similar Metal-Forming Machine." Shiraishi and Schneider both disclose movement of a cross bar along a curved path from a rest position between stations to a first press station. The work piece is transferred from the first press station to a second press station over a curved path and the cross bar returns to the rest position between press stations. The cross bar stays in the rest position during each pressing operation.
The Schneider patent also shows cross bar assemblies with carriages formed with low-mass construction to allow increased acceleration and thus a higher operating speed for the associated transfer press. Schneider also discloses idle stations disposed between each of the press stations to help reorient the work piece for subsequent processing. Although the idle stations may allow shortening the transfer movements of the work piece, they also introduce a delay by adding extra stations. Also, the idle stations require additional tooling. The idle stations add to the possibility of damaging a work piece by doubling the number of times each work piece is handled.
While changing the dies at a press station to fabricate a different part, it is often necessary to replace either the complete cross bar assembly or the holding devices on the associated cross bars to accommodate work pieces with configurations corresponding with the new die sets. Also, one or more holding devices may need to be replaced as part of normal maintenance and repair of the associated transfer press. Typically, changing holding devices in prior transfer presses required either removing the complete cross bar assembly or at least the respective cross bar from the associated transfer press. Therefore, replacing the complete cross bar assembly and/or holding devices often resulted in substantial downtime for the associated transfer press.